In the agitators of this type known from German Disclosed Patent Application 26 43 560, a first portion of the stirring wing which acts in the one sense of rotation to direct the material in the tank upwardly is followed, viewed in the radial direction, by a second stirring wing portion which directs the material in the container downwardly. These sections are inclined at a relatively big angle relative to the sense of rotation. However, these known stirring wings are not suited for use in grape mash fermentation tanks as these known stirring wings do not achieve a satisfactory degree of mixing of the grape mash as these stirring wings will in most of the cases entrain the mash in a circular movement. To overcome this last-mentioned drawback, one generally provides the inner wall of the container with rigid hold-ups which project right into the area of the mixing wings. But in most of the cases, these hold-ups obstruct the thorough vertical mixing of the grape mash which in fact is composed of rape, yeast, juice and grapeseeds, i.e. substances of very different densities. Very unsatisfactory results are obtained with these known agitators in particular when mixing grape mash after the beginning or completion of the process of fermentation.
On the other hand, if the angle of inclination of the stirring wings known from German Disclosed Patent Application No. 26 43 560 is selected relatively small, the tendency of the agitator wing to entrain the grape mash in a rotary movement is indeed reduced, but vertical lifting of the material to be mixed is unsatisfactory so that only a very low and unsatisfactory degree of mixing of the grape mash in the vertical direction is achieved. Still, one cannot at desire increase the dimensions of slightly inclined mixing wings in the direction of rotation so as to achieve sufficient lifting of the material to be mixed, because such an increase of the surface of the mixing wings would in turn increase a tendency for the material to be mixed to be entrained in a rotary movement rather than to be lifted.
To achieve vertical mixing of the grape mash, Patent Application No. P 30 12 829 proposes to provide a helically wound sheet steel strip at a certain distance around the vertical shaft, which strip is held at the vertical rotating shaft by means of bars to rotate together with the shaft and convey the material in the one sense of rotation upwardly--if the material does not rotate together with the relatively wide sheet steel strip. Immediately adjacent to the circumference of the shaft, there is provided a second sheet steel strip of inverse helical shape so that in the one sense of rotation the material in the tank is pressed upwardly by the outer helical strip and downwardly by the inner helical strip. This design is extremely complicated and expensive, the sheet steel strips are difficult to clean and have a tendency, due to their large surface, to entrain the grape mash in their circular movement. Moreover, the degree of mixing of the grape mash is considerably inferior to what one could expect, in particular when red grape mash is to be mixed.
But it is just the mixing of red grape mash during fermentation that creates special problems. In tanks without agitators, the rapes tend to concentrate on the surface and to form a very solid grape-cake. But if red wine is to get a deep colour, the rapes must remain in intimate contact with the juice during fermentation. On the other hand, however, it is necessary that mixing be carried out with utmost care.